RFC: Documentation System

Ok, so this has ended up evolving into a full doc system. Functions and macros can be documented with
```julia
@doc """
Description here
""" ->
function foo() ...
```
(prettier syntax forthcoming)

Documenting multiple methods of a function will concatenate the doc strings together, in the order the methods were originally defined.

By default `@doc` treats unadorned strings as equivalent to `md""` (i.e. markdown syntax) but you can actually put anything before the arrow to associate it with the function.

Docs can be queried with e.g.
```julia
doc(Docs.doc)
@doc Docs.doc
@doc @doc # For macros
@doc text""
```
(and you can try these out on this branch)

---
*Original:* Here's a first pass at getting Markdown.jl into Base. This will us allow us to display Julia's doc strings in the terminal and rich environments; see the [readme](http://github.com/one-more-minute/Markdown.jl) for some examples.
Though I'm open to being corrected, I don't remember Michael expressing any concern about parse times or the need to parse lazily. So I'm not sure where that's come from, but it sounds like it's not something to worry about.